Similar kinds of blood sampling devices are known from EP-A2-0 302 752 and WO 88/01846. When an infusion tube or a catheter has been inserted into a vein or artery of a patient, such as for a drip infusion and/or an indwelling direct blood pressure measuring device, and a blood sample is to be taken for analysis purposes, this sample is taken from the infusion tube at a point that is accessible from the outside. To this end, the apparatus of the prior art provide for sampling points, which are connected with a channel of the infusion system and are accessible from the outside by means of a plunger-type syringe.
Before blood can be sampled from this system, it must first be ensured that only undiluted blood of the patient is supplied, excluding any components of the infusion solutions or anticoagulants used for the purposes of direct blood pressure measurements. To this end, WO 88/01846 provides for two sampling points. The sampling point lying closer to the patient (anterior) is used for the actual withdrawal of the blood sample. The sampling point lying farther away from the patient (posterior) is used for temporary removal of infusion solutions from the system so that only undiluted blood is present at the anterior sampling point. Following withdrawal of the blood sample at the anterior sampling point, the quantity of infusion solution and blood taken at the posterior sampling point is reintroduced into the system.
With WO 88/01846, the removal and the reintroduction at the posterior sampling point are carried out by means of a conventional syringe, which has a needle that penetrates the otherwise impermeable seal plug. However, this involves complicated manipulation and entails the risk that contamination, germs and other morbific agents may enter the system through the injection needle. There is also the risk that hospital personnel can injure and infect themselves when they remove the needle; this has already led to a high number of hospital personnel being infected with AIDS.
It has been suggested to use a closed system that is not accessible from the outside when the posterior sampling point is designed as an indwelling intermediate accumulator equipped with a plunger/barrel arrangement and integrated into the infusion or catheter system (cf. brochure of the firm of Baxter, entitled "New Vamp; a closed system for easier, safer blood sampling from invasive lines" and the one of the firm of Pfrimmer Viggo GmbH & Co. KG entitled "Saw draw; geschlossenes Blutentnahmesystem mit Statham-Einmal-Druckwandler DTX/plus (i.e. Closed Blood Sampling System with Disposable Statham Pressure Transducer, DTX/Plus).
The sterility of the intermediate accumulator cannot always be guaranteed with these known blood sampling arrangements, particularly when blood is to be drawn from a patient a number of times one after the other. If a conventional plunger-type syringe is used, such as in WO 88/01846, bacteria and other kinds of germs can be entrained into the system when the plunger is pulled out of and then re-inserted into the cylinder or housing chamber.